Let me explain. Many crooks want money. A lot of illegal activities generate what looks like value but it's hard to turn into money and unless you do it's useless. A stolen television is valuable but why would I pay 60% of retail price to some geezer in a lock-up at 2am for what is obviously a stolen TV? The Fence buys stolen things from crooks for a fraction of their value, then operates a seemingly legitimate business which sells those things to ordinary punters to collect the difference.
Being a Fence is a crime. "Receiving stolen goods" we call it in most places. You're guilty if you know or should reasonably have known that the goods you got were stolen.
The argument against G2A is that logically the stuff they're buying for cheap can only exist if it's being obtained by crooks. A week after a game is released for $25 the _only_ reason somebody wants to sell 100 codes for $5 each is that they used stolen credentials to get those codes and are cashing out.